Shape-controlled nanostructure synthesis of noble metals, such as silver and gold in particular, has attracted a great deal of attention in recent years because of their unusual optical properties known as localized surface plasmon resonance (LSPR), as well as their novel chemical, electronic, and catalytic properties. Consequently, a broad range of intriguing applications capitalising on such unique nanostructures properties have emerged in the field of photonics, catalysis, biological and chemical sensing, surface-enhanced Raman scattering (SERS), metal-enhanced fluorescence (MEF), and energy conversion.
The need to produce nanoparticles (NPs) with finely-tuned optical properties has led to enormous research efforts on developing reliable routes to synthesize noble metal NPs with controllable sizes and shapes, such as sphere, rod, wire, prism, cube, octahedron, star, icosahedron, and bipyramid.
Bimetallic silver (Ag) and gold (Au) nanocrystals are particularly attractive because they possess broader range of plasmon tunability and versatile surface functionality as compared to the individual unit of Ag or Au nanocrystal. By combining Au and Ag into core-shell structures, the resultant LSPR signatures can be controlled by not only varying the size and shape of the core but also the shell thickness. The close lattice match between Au and Ag (<0.3% mismatch) plays a key role in achieving conformal epitaxial growth. For example, Au@Ag core-shell nanocrystals with various morphologies have been synthesized through an epitaxial growth process involving conformal Ag deposition on the surface of Au seeds.
However, the formation of a bimetallic nanostructure with a Ag core and a Au shell remains challenging due to the significant etching of the Ag core by Au salt precursors, which is known as galvanic replacement. In particular, when the core is a Ag nanoprism with very small thickness (<10 nm), the tips and edges are so vulnerable to oxidation and the flat (111) faces tend to be preferentially etched through the galvanic process. For example, by seeding with Ag nanoplates, bimetallic Ag@Au nanostructures with non-uniform gold coating and pinholes in the structure were produced. In another example, rounded-tip triangular Ag@Au core-shell nanostructures with corrugated gold shells were produced by using cetyltrimethylammonium bromide (CTAB) as the surfactant to mitigate Ag prism etching. However, the presence of CTAB led to severe tip truncation of the Ag prism cores. More seriously, the strong passivation of gold shell surfaces by CTAB induces tremendous difficulties when further surface modification is needed for application purposes.
Therefore, there remains a need to provide for a method for forming a bimetallic core-shell nanostructure that overcomes, or at least alleviates, the above problems.